US authorities probing the shooting spree at Washington's Navy Yard, which left 13 people dead and eight injured, said on Monday that a lone gunman carried out the killings.

"We don't have any evidence or indication at this stage that there was another shooter, even though we haven't
completely ruled that out," said Washington mayor Vincent Gray.

Police chief Cathy Lanier stressed that: "we do now feel comfortable that we have the single and sole person responsible for the loss of life inside of the base today. If anything changes as we continue to go forward, we certainly will make that available to the public," she told a briefing hours after authorities said they were searching for two suspects and urged Washington residents to stay in place until the search ended.

Lanier announced that the "shelter-in-place" warning to residents had been lifted as the authorities were satisfied there were no suspects on the loose.

Police have identified the alleged lone gunman as Aaron Alexis of Forth Worth, Texas, who served in the Navy from 2007 to 2011 before becoming a defense subcontractor for computer giant Hewlett-Packard.

He was killed in an exchange of gunfire with police.

Separately, a US defense official told AFP the suspect had gained access (to Navy Yard) through legitimate means.